Transporting electricity by means of high voltage superconductor cables enables high currents to be transported while using cable sections that are much smaller than is possible with conventional cables made of resistive electrical conductors, while simultaneously limiting electrical losses along the cable, in particular losses due to the Joule effect, since that phenomenon is extremely weak with superconductivity. In order to reduce losses, electricity is transported in multiphase alternating current (AC) form, generally three-phase AC, with one phase cable being dedicated to each of the phases (so with three-phase AC, there are three separate phase cables).
A “cold-dielectric” superconductor phase cable is constituted by a central electrical conductor constituted at least by: a superconductive portion (referred to below as the “central superconductor”), electrical insulation surrounding said superconductor (referred to below as the “dielectric”), a shield surrounding said dielectric, which shield may be constituted in full or in part by superconductors, and a cryogenic enclosure or “cryostat” surrounding said shield. Said cryostat is generally constituted by two concentric coverings that are thermally insulated from each other (by a vacuum at 10−5 millibars (mbars), for example). A cryogenic fluid contained inside the inner covering of the cryostat cools the central superconductor through the dielectric (whence the term “cold dielectric) down to the temperature at which the superconductor is in a superconductive state (with this temperature being of the order of −196° C., for example, for so-called “high temperature” super-conductors).
For safety reasons, the presence of a shield in a cable is mandatory once the voltage applied to the cable reaches a certain value (e.g. 1 kilovolt (kV) in France). These shields are connected to a ground potential, generally to earth. This avoids any risk of electrocution in the event of coming accidentally into contact with the cable, e.g. because a person is digging in the ground in which an electric cable is buried.
With cold-dielectric superconductor cables, currents of similar magnitudes are caused to flow both in the central superconductor and in the shield (particularly if the shield is constituted in full or in part by a super-conductor). For high-voltage cables, the magnitude of this current can be high (e.g. 2400 amps (A)) and it is therefore not possible to envisage connecting the shield directly to the earth. The solution consists in interconnecting the shields of the phase cables. The resulting current is the vector sum of the phase currents, so the magnitude of the resulting current is zero or nearly zero and it can therefore be connected to the earth. For three-phase AC, two cables suffice to interconnect the shields of all three phases, and a resistive cable (e.g. a copper cable) is generally used for making the connection to the earth after vector canceling of the currents. Nevertheless, interconnecting the shields by a resistive cable leads to thermal and electrical loses by the Joule effect, and thus to an increase in the quantity of cryogenic fluid consumed for cooling the superconductor, and leads to a drop in the electrical efficiency of the installation as a whole. Furthermore, if thermal losses are too great, the rise in temperature around the resistive link can prevent the cryogenic fluid from being effective in cooling the superconductor in the vicinity. The superconductive portions can then switch from a superconductive state to a state of ordinary conductivity, thereby worsening the drop in the electrical efficiency of the installation.